Wavelength division multiplexed systems, in which multiple channels are carried at different wavelengths on the same optical fiber, require adjustable output power to address problems such as optical crosstalk between channels and power balancing of optical signals for optical amplifiers. It is common today to control the output power of a semiconductor laser diode to maintain a constant operational output level, for example, 0 dBm. The constant output power laser diode is used in combination with an optical attenuator to provide the adjustable output power that is needed. The type of optical attenuator can be either fixed or variable attenuation. The fixed attenuation type is neither field adjustable nor remotely controllable. The variable attenuation type is large and expensive and can require additional power sensing circuitry.
There is a need for an approach to controlling the output power of laser diodes that is less costly and less bulky than those that require external optical attenuators. There is also a need for a power control mechanism that takes into account the relationship between temperature and wavelength in the operation of laser diodes.
An apparatus and method of the present approach provides for electrical control of the laser output power without the need for a costly and bulky optical attenuator. The present approach further provides wavelength control to compensate for the relationship between laser diode operating temperature and wavelength.
Accordingly, a control circuit for a laser diode includes a power controller and a wavelength controller. The power controller adjusts a bias current to the laser diode to change the power output of the laser diode. The power change can have a corresponding wavelength shift effect on the nominal operating wavelength of the laser diode. The wavelength controller compensates for the wavelength shift such that the laser diode maintains operation at the nominal wavelength.
In an embodiment, the power controller includes a bias current source that provides an adjustable bias current to the laser diode. A power monitor loop includes a backfacet diode for monitoring the laser diode power output to provide a power monitor signal. A power control signal added to the power monitor signal provides a power adjust signal. The bias current source adjusts the bias current responsive to a difference between a power reference voltage input of the bias current source and the power adjust signal.
In an embodiment, the wavelength controller includes a temperature control circuit that provides a control current to a thermoelectric element for controlling the temperature operation point of the laser diode. A temperature monitor loop includes a temperature sensor for monitoring the temperature operation point to provide a temperature monitor signal. A wavelength compensation signal added to the temperature monitor signal provides a wavelength control signal. The temperature control circuit adjusts the control current to the thermoelectric element responsive to a difference between a temperature reference signal and the wavelength control signal.
The wavelength compensation signal may be proportional to the power control signal.
In an alternate embodiment, the wavelength controller includes an etalon element for wavelength compensation.
In one aspect of the invention, a control circuit includes a power controller for adjusting a bias current to a laser diode to change the power output of the laser diode, the change in power having a corresponding wavelength shift effect on the nominal operating wavelength of the laser diode and a monitoring circuit for sensing the bias current to the laser diode and for generating an output signal in response to the sensed bias current. The control circuit further includes a wavelength controller which receives the output signal from the monitoring circuit and in response to the output signal compensates for the wavelength shift such that the laser diode maintains operation at the nominal wavelength.
Embodiments of this aspect of the invention may include one or more of the following features. The monitoring circuit includes a sensing resistor. The power controller includes a bias current source that provides an adjustable bias current to the laser diode and has a power reference voltage input. The power controller also includes a power monitor loop having a backfacet diode for monitoring the laser diode power output to provide a power monitor signal, and a power control signal added to the power monitor signal to provide a power adjust signal. The bias current, source adjusts the bias current responsive to a difference between the power reference voltage input and the power adjust signal.
The wavelength controller includes a temperature control circuit that provides a control current to a thermoelectric element for controlling the temperature operation point of the laser diode and having a temperature reference voltage input and a temperature monitor loop including a temperature sensor for monitoring the temperature operation point to provide a temperature monitor signal a wavelength compensation signal added to the temperature monitor signal to provide a wavelength control signal. The temperature control circuit adjusts the control current to the thermoelectric element responsive to a difference between the temperature reference voltage input and the wavelength control signal. The wavelength compensation signal is proportional to the sensed bias current.
The control circuit can further include the laser diode and a modulator for modulating the output of the laser diode.
In another aspect of the invention, a method of controlling a laser diode includes the following. A bias current to the laser diode is adjusted to change the power output of the laser diode, the power change having a corresponding wavelength shift effect on the nominal operating wavelength of the laser diode. The level of bias current to the diode is sensed. In response to the sensed level of bias current, compensating for the wavelength shift such that the laser diode maintains operation at the nominal wavelength.
Embodiments of this aspect of the invention may include one or more of the following steps. Adjusting the change of power output includes monitoring the laser diode power output to provide a power monitor signal, adding a power control signal to the power monitor signal to provide a power adjust signal, and adjusting the bias current responsive to a difference between a power reference voltage signal and the power adjust signal.
Compensating for the wavelength shift includes providing a control current to a thermoelectric element for controlling the temperature operation point of the laser diode, monitoring the temperature operation point to provide a temperature monitor signal, adding a wavelength compensation signal to the temperature monitor signal to provide a wavelength control signal, and adjusting the control current to the thermoelectric element responsive to a difference between a temperature reference signal and the wavelength control signal.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.